The present invention relates to the method for manufacturing a Metal-Semiconductor-Metal (MSM) photodetector using a HEMT structure incorporating a low-temperature grown semiconductor. MSM photodetectors are widely used as a key component for optical receivers in ultra-high speed optical communication systems and for optoelectronic transducers in microwave/millimeter-wave optical communication systems.
Photodetectors perform the function of converting optical signals to electrical signals. MSM photodetectors are one of many candidates for ultra-high speed photodetectors and are widely used due to their excellent speed performance. They are also easy for monolithic integration (opto-electronic integrated circuit: OEIC) with electronic devices including FETs (field effect transistors) and HEMTs (high electron mobility transistors) since they are planar in structure.
The epitaxial layer structure and schematic cross section of conventional MSM photodetectors are illustrated in FIG. 1a and FIG. 1b, respectively. The cross section and the energy band diagram of conventional MSM photodetectors are also illustrated in FIG. 2a and FIG. 2bb, respectively. Here, the first epitaxial layer 10 is a semi-insulating substrate, the second epitaxial layer 12 is a photo-absorption layer, and the third epitaxial layer 14 is a Schottky barrier layer.
In general, the Schottky barrier layer is composed of an n-type semiconductor having a larger band gap energy than that of the photo-absorption layer. The Schottky barrier layer is transparent to optical signals that are absorbed in the photo-absorption layer and plays a role of reducing the leakage current (or dark current) in MSM photodetectors. The photo-absorption layer is composed of a lightly n-doped semiconductor that has a lower band gap energy than that of the Schottky barrier layer and absorbs the photons incident upon the photodetector, generating and transporting electron-hole pairs.
The two main factors that determine the speed performance of a photodetector are the RC time constant (xcfx84RC) that is dependent on the resistance and capacitance of the photodetector and the transit time (xcfx84T) that is the time taken for an electron or a hole to reach the metal electrode via a photo-absorption layer along the applied electric field (the transport length L and the direction of the electric field are shown in FIG. 1b).
In order to maximize the speed performance of a photodetector, these two factors must be optimized. In practice, the RC time constant (xcfx84RC) of an ultra-high speed photodetector is designed to have a significantly smaller value than the transit time (xcfx84T) and thus the speed performance is determined by the transit time of photo-generated electrons and holes.
Photons incident on the photo-absorption layer of a photodetector generate electron-hole pairs. The photo-generated electrons and holes travel in opposite directions according to the direction of electric field applied and reach the Schottky metal electrode, subsequently constituting electron and hole currents, respectively.
In general, the speed of electrons and holes in most of semiconductors is different. In fact, the speed of electron is much faster than that of hole. This is due to the difference in their effective mass. For example, in the case of In0.53Ga0.47As that is lattice-matched to InP substrate and used for detecting an optical signal having a wavelength of 1.5 xcexcm, the effective mass me of an electron is 0.041 mo whereas the effective mass mh of a hole is 0.46 mo (Here mo is the mass of a free electron). This shows that the effective mass of a hole is 10 times heavier than that of an electron.
FIG. 3 shows the photo-current response of conventional MSM photodetectors when an impulse-type optical signal is incident upon them. The total current here consists of electron and hole currents. The different pulse width of the electron current and the hole current is due to difference in speed of electrons and holes in semiconductors.
Consequently, the speed performance of MSM photodetectors, more specifically, the frequency bandwidth is determined by the speed characteristic of the holes. This can be represented quantitatively as follows.
If the time taken for an electron and a hole to travel through a photo-absorption layer (L in FIG. 1b) is xcfx84e and xcfx84h, respectively, then the frequency bandwidth of the photodetector B can be represent as;
B=xc2xdxcfx80(xcfx84e+xcfx84h)xe2x80x83xe2x80x83[Mathematical Equation 1]
Bxcx9c1(2xcfx80xcfx84h) since xcfx84 less than xcfx84h
As a result, it can be seen that the speed performance of MSM photodetectors is determined by the speed characteristic of holes. Accordingly, in order to improve the speed performance of MSM photodetectors the photocurrent response due to heles has to be modified.
In general, the photocurrent response of an MSM photodetector consists of electron current and hole current originating from the equal number of electrons and holes generated by photons. The hole current acts as a limiting factor for the speed performance of an MSM photodetector due to the low hole mobility (or speed). This speed performance limitation can be overcome by suppressing the magnitude of the hole current, making the electron current a dominant part of the total photocurrent of an MSM detector. Suppression of the hole current can be achieved by reducing the number of holes reaching the metal electrode.
FIG. 2b illustrates the operation principle of a conventional MSM photodetector. In FIG. 2b, the holes generated in the photo-absorption layer move upward (toward the metal electrode) due to the applied electric field. Since the direction of the electric field is aiding transport of the holes to the metal electrode the probability of the holes being converted to photocurrent increases. On the contrary, the electrons move downward (away from the metal electrode) due to the applied electric field. Since the direction of the electric field is hampering transport of the electrons to the metal electrode the probability of the electrons being converted to photocurrent decreases.
The object of the present invention is to improve the speed performance of an MSM photodetector by reducing the magnitude of the hole current. Reduction in the magnitude of the hole current can be achieved by deducing the probability of the holes generated in the photo-absorption layer eventually reaching the metal electrode. This objective is achieved by using an energy band structure constructed by a modified HEMT structure and a low-temperature grown compound semiconductor.
In order to achieve the object of the present invention, the method for manufacturing an MSM photodetector using a GaAs-based delta-doped HEMT structure according to the present invention comprises the steps of: forming an undoped AlxGa1-xAs (0xe2x89xa6xxe2x89xa60.4) buffer layer which is grown on a GaAs semi-insulated substrate; forming an undoped low temperature grown InxGa1-xAs (0xe2x89xa6xxe2x89xa60.3) epitaxial layer on said AlxGa1-xAs buffer layer; forming a p-type GaAs first photo-absorption layer which is grown on said low-temperature grown InxGa1-xAs layer; forming an undoped GaAs second photo-absorption layer which is grown on said GaAs first photo-absorption layer; forming an undoped AlxGa1-xAs (0xe2x89xa6xxe2x89xa60.4) first barrier layer which is grown on said undoped GaAs second photo-absorption layer; forming an n-type delta-doped AlxGa1-xAs (0xe2x89xa6xxe2x89xa60.4) second barrier layer which is grown on said undoped AlxGa1-xAs (0xe2x89xa6xxe2x89xa60.4) first barrier layer; and forming an undoped AlxGa1-xAs (0xe2x89xa6xxe2x89xa60.4) third barrier layer which is grown on said n-type delta-doped AlxGa1-xAs (0xe2x89xa6xxe2x89xa60.4) second barrier layer.